move assertions out of system (#19599)

compiler/types.nim

@@ -13,6 +13,9 @@ import
   intsets, ast, astalgo, trees, msgs, strutils, platform, renderer, options,
   lineinfos, int128, modulegraphs, astmsgs
 
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
 type
   TPreferedDesc* = enum
     preferName, # default